This invention relates generally to apparatus for correlating two time-varying input signals, and, more particularly, to a simultaneous-delay correlator apparatus that delays the two input signals simultaneously and produces a correlation envelope representing the correlation of one input signal with multiple delayed and advanced versions of the other input signal.
Apparatus of this particular kind are useful in many applications. One such application is as a component of a spectrometer that measures the cross power spectrum of two time-varying input signals. In the case of microwave signals, the apparatus can be implemented in either waveguide form or strip line form.
One typical simultaneous-delay correlator apparatus of this kind is depicted in FIG. 1. It includes two parallel transmission lines 6 and 7, one for each input signal. Each transmission line includes a plurality of directional couplers 8 spaced along its length, to produce multiple delayed versions of each input signal. Prescribed pairs of these delayed signals are correlated with each other to produce a plurality of correlation signals, i.e., a correlation envelope, representing the cross correlation of the two input signals. In FIG. 1, each such individual correlation is shown being performed by a simple square-law detector 9, which provides a real correlation. Alternatively, a complex correlation can be provided using a combination of mixers and phase shifters. A fast-Fourier transform device (not shown) can then operate on the plurality of correlation signals, to produce a real or complex cross power spectrum of the two input signals. In the special case where the two input signals are the same, the plurality of correlation signals represent an autocorrelation of the signal and the fast-Fourier transform device produces that signal's power spectrum.
Although the simultaneous-delay correlator apparatus described briefly above has proven generally effective in correlating two time-varying input signals, it is believed to be unduly complex and massive for many applications. In addition, construction and alignment of the apparatus can sometimes be difficult, which leads to low product repeatability. It should therefore be appreciated that there is a need for a simplified simultaneous-delay correlator apparatus that is smaller in size and easier to construct and align than previous correlator apparatus. The present invention fulfills this need.